A restaurant run by the sons of one of Ukip's top candidates in next week's European elections was fined last year for employing illegal immigrants, it has emerged.

Amjad Bashir, Ukip's spokesman for small businesses and one of the party's few non-white members, confirmed on Friday that four workers at Zouk, his family's curry house in Manchester, were arrested last June for what he called minor immigration offences. The upmarket restaurant was subsequently fined, he said, insisting that the penalty was nowhere near the £70,000 reported by the Times.

However, Bashir would confirm neither exactly how much the restaurant was fined nor what the workers or firm had done to break immigration law. The 61-year-old, who is very likely to be elected in Yorkshire next Thursday, said the business was mounting a legal challenge against the sanctions. He blamed government bureaucracy for the legal breach, saying businesses were unfairly forced to do the job of the UK Border Agency.

Zouk is run by Bashir's two sons, Tayub, 32, and Mudassar, 31. They also run a sister restaurant in Bradford, Zouk Tea Bar & Grill, which their father opened in October 2006.

Bashir claims he had stepped down from the family business before the raid. Records kept by Companies House show he resigned as a director of Zouk Group Limited and four related companies on 6 May 2013, a month before the visit from immigration officials. But the resignation was not filed electronically with Companies House until 3 June, two days after the raid. Zouk featured prominently in Ukip's last party political broadcast.

A spokeswoman for the Home Office said on Friday that seven immigration offenders were arrested at Zouk on 1 June 2013 but could not confirm how many were eventually found guilty.

In a phone interview on Friday, Bashir blamed the government for forcing too much bureaucracy on firms, insisting it was the border agency's job to make immigration checks, not that of businesses.

"This is a dereliction of duty on the part of the government. What I mean is, border control and immigration control is the role of the government. What's it got to do with businesses? Are we trained in immigration law?

"They are asking businesses to carry out their work, not just in immigration but almost everything – tax collection, VAT collection. All sorts of legislation has been put on businesses, which is not right. This is why I became a politician, to say this is the role of government, not business. Businesses are there to create jobs. They are there to make money for themselves and their employees."

Bashir said he set up Zouk in Manchester in 2009 but "never had a hands-on role" and only recently became aware of the immigration raid and subsequent fine.

He denied it was hypocritical for his family firm to employ illegal workers when his party took such a tough line on immigration.

"It's a democratic country. If laws are imposed we've got to challenge them. We can't just say, 'That's fine' … Why are there 120,000 laws already coming from the EU and growing every day? And more laws coming out from parliament? The bureaucracy is killing businesses. Bureaucracy and taxation is killing businesses. So it's hypocritical of the government to ask businesses to do this."

He said the company didn't employ illegal immigrants. "Hundreds of thousands of businesses are raided, or been visited. Hundreds of thousands, right. And it's for minor infringements, right. It's not that they are deliberately employing illegal people, not at all."

Speaking on LBC Radio, Nigel Farage, defended Bashir. "Be a little bit careful on that story; he founded a business of which he is no longer a director of, his sons run it, they have got a big row going on. He's resigned as a director but he wasn't responsible for the day-to-day running of it. However, I've spoken to him this morning; he said they made checks and they are in dispute with the immigration authorities and they've gone to appeal."

Bashir is second on Ukip's list of candidates for the European election in Yorkshire. Anthony Wells, a polling expert at YouGov, said Ukip was very likely to win at least two of the six seats being contested in the region.

At the last European poll in 2010, Ukip won one seat in the county: Godfrey Bloom become the MEP. He is currently serving as an independent MEP after being thrown out of the party for making disparaging remarks about women and foreigners from "bongo bongo land".

Bloom is not standing again. Nor is Andrew Brons, originally elected on a British National party ticket, but also now an independent after falling out with Nick Griffin, the leader of the far-right party. Ukip is expected to win both those seats plus a possible third in Yorkshire.